1. Field of the Invention
The present invention generally concerns photocatalytic particles and aggregates and coatings, especially as may incorporate nanoparticulate titanium dioxide, and to processes for the production and the use thereof.
The present invention further generally concerns photocatalytic materials as are effective for, inter alia, killing microorganisms, including algae and bacteria, on contact in the presence of light in the visible or ultraviolet wavelengths. More particularly as regards these photocatalytic materials, the present invention concerns (1) composite photocatalytic materials in the form of particles and other bodies with surfaces which particles and bodies have (1a) cores nondeleterious to photocatalytic action coupled with (1b) photocatalytic surfaces; and (2) liquids, aggregates and solids incorporating such (1) photocatalytic materials.
2. Description of the Prior Art
2.1 Photocatalytic Coatings, Especially as May Incorporate Nanoparticulate Titanium Dioxide
A first aspect of the present invention will be seen to concern the production, and use, of photocatalytic coatings, especially as may incorporate nanoparticulate titanium dioxide.
For the purposes of the present invention, nanoparticulate titanium dioxide coating (xe2x80x9cnano-coatingxe2x80x9d) is taken to be surface coatings of rutiles, anatases and amorphous titanium dioxide having a particle size of 1 to 100 nm, preferably of 1 to 50 nm, and more preferably of 1 to 10 nm, or titanium dioxide having the above-stated particle size dispersed and adhering on a surface.
2.1.1 Applications for Titanium Dioxide Nano-coatings
Applications for such titanium dioxide nano-coatings include the following. Pigmentary particles may be coated with titanium dioxide to impart improved U.V. absorption or opalescent effects. In this application the light transparency of the titanium dioxide due to the small particle size is a particularly desirable characteristic of the nano-coating.
Titanium dioxide nano-coatings may be applied to building materials as a photocatalytic coating providing anti-fouling benefits. Photocatalytic surfaces so created are particularly useful in public areas such as rest rooms and hospitals to reduce bacterial contamination.
A titanium dioxide nano-coating may be applied as a photocatalytic coating to a waste water treatment apparatus.
A titanium dioxide nano-coating may be applied to both powders and continuous surfaces as a coating for absorption of U.V. radiation,
A titanium dioxide nano-coating may be applied to a surface as a flame retardant surface.
A titanium dioxide nano-coating may be applied to a surface as a support layer in a dye solar cell.
The use of titanium dioxide nano-coatings is, however, currently still restricted by the fact no economic process is known which is capable of producing nano-coatings comprised of the stated particle size on an industrial scale. The present invention deals with this issue.
2.1.2 Sol/gel Coatings of Nano-particulate TiO2 
The most important previous methods for the formulation of nano-particulate TiO2 coatingsxe2x80x94also known as titanium dioxide nano-coatingsxe2x80x94may be grouped together under the superordinate term of xe2x80x9csol/gel coatingsxe2x80x9d. Sol/gel coatings have been described in many journal articles and patents. Nano-particles of TiO2 in the sol/gel form are attracted to surfaces by van der Waals"" forces and may be further anchored to surfaces by stronger chemical bonds 1% such as fusion bonds.
Sol/gel materials are desirable because, when applied as a film to surfaces, these nano-particulate suspensions create the thinnest surface coatings, disperse evenly, and have good adhesion properties.
As discussed in U.S. Pat. No. 5,840,111, the sol/gel coatings are generally formulated using the alkoxide method, i.e. the carefully controlled, frequently base- or acid-catalyzed hydrolysis of metal alkoxides and similar molecular precursors in mixtures of water and one or more organic solvents. The solvent used is generally the same alcohol as is the basis of the alkoxide. One disadvantage of this previous process is that costly educts and complicated processing are required. Moreover, the products have an undesirably elevated carbon content.
Originally developed for silicon compounds, the alkoxide method is increasingly also being used for the synthesis of nano-titanium dioxide in accordance with the equation
Ti(OR)4+2H2Oxe2x86x92TiO2+4 ROH 
See, for example, J. Livage, Mat. Sci. Forum 152-153 (1994), 43-54; J. L. Look and C. F. Zukoski, J. Am. Ceram. Soc. 75 (1992), 1587-1595; WO 93/05875.
It is frequently possible to produce monodisperse particles, i.e. particles having a very narrow particle size distribution, by appropriate selection of the reaction conditions, permitting production of homogeneous particles ranging in diameter from some micrometers down to a few nanometers. One example of such a special processing method is working in microemulsions, by which means it is possible to limit particle size. See, for example, D. Papoutsi et al., Langmuir 10 (1994), 1684-1689.
The educts for virtually all sol/gel reactions for the production of TiO2 nano-coatings, whether by conventional or microemulsion methods, are titanium alkoxides (Ti(OR)4), the alkyl residues R of which conventionally contain 2 to 4 carbon atoms. However, due to the high price of these alkoxides and particular handling requirements (protective gas, strict exclusion of moisture in order to prevent premature hydrolysis), the stated reactions have not been considered for a large scale industrial process.
Still furthermore, working in microemulsions has the disadvantage that, due to the frequently low concentration of the reactants, (i) the space/time yield is low and (ii) large quantities of water/solvent/surfactant mixtures are produced which must be disposed of.
An alternative, a non-hydrolytic sol/gel manufacturing process has recently been proposed which involves reacting metal halides with oxygen donors such as ethers or alkoxides. See S. Acosta et al., Better Ceramics through Chemistry VI (1994), 43-54.
2.1.3 Chemical Vapor Reaction Processes for the Production of TiO2 as May be Used in Nano-Coatings
Yet another group of methods for the production of ultra-fine titanium dioxide particles comprises the so-called CVR (chemical vapor reaction) processes, which are based upon the reaction of vaporizable metal compounds (generally alkoxides) with oxygen (air) or steam in the gas phase. This process is described, for example, in U.S. Pat. No. 4,842,832 and Europe patent no. EP-A 214 308. While small quantities of powders produced using such processes are presently (circa 2000) commercially available, they are extremely expensive.
2.1.4 Industrial Processes Producing TiO2 Undesirably Coarse for Use in Nano-Coatings
Of the hitherto known processes performed on a large industrial scale for the production of finely divided (sub-pigmentary) titanium dioxide, none yields a product comparable in terms of fineness and transparency with sol/gel materials. These industrial processes include hydrolysis of TiCl4 as is shown in Great Britain patent no. GB-A 2 205 288; production of rutile nuclei in the sulfate process as is shown in Europe patents nos. EP-A 444 798 and EP-A 499 863; and peptisation with monobasic acids of titanium dioxide hydrate which has been washed free of sulfate as is shown in Europe patent no. EP-A 261 560 and also in U.S. Pat. No. 2,448,683.
It is also known from U.S. Pat. No. 5,840,111 to react a solution comprising sulfuric-acid and titanyl sulfate by adding an alkaline-reacting liquid such that the alkaline liquid is present in a stoichiometric deficit relative to the xe2x80x9cfree sulfuric acidxe2x80x9d (which is the total sulfur content minus that proportion bound in the form of foreign metal sulfates). The resultant solution is then flocculated by adding a monobasic acid. This process is inefficient because a significant portion, approximately 50%, of the titanyl sulfate does not react acidically with the stoichiometrically deficient alkaline liquid so that a significant portion, approximately 50%, of the potential TiO2 product is left in solution in the form of titanyl sulfate.
It is also known from the literature to hydrolyse TiCl4 under hydrothermal conditions, wherein depending upon the reaction conditions (concentration, temperature, pH value, mineralisers), nano-anatases and nano-rutiles are obtained. See H. Cheng et al., Chem. Mater. 7 (1995), 663-671. However, due to the complicated processing requirements, it is doubtful that a commercially viable product may be obtained using this method.
2.1.5 Objects of the Present Invention as Regards the Production and Use of Coatings, Particularly Nanoparticulate Titanium Dioxide Coatings
It is thus a primary object of the invention to produce at high yield a well-adhering thin, uniform, transparent titanium dioxide nano-coatingxe2x80x94in which nano-coating is present titania nanoparticlesxe2x80x94and to provide a process for the application thereof. The processes for each of (1) the production and (2) the application of nano-titanium dioxide coatings should be economically viable, and would preferably entail only relatively simple and foolproof conventional processing requirements that, when conducted at an industrial large scale, will reliably produce a titanium dioxide nano-coating product fully having the most favorable thinness, uniformity, and adhesion properties of the best sol/gel films.
2.2 Prior Art Regarding the Application of Photocatalytic Coatings
The previous sections 2.1 have discussed prior art, and the deficiencies of the prior art, in the economical industrial scale production of photocatalytic coatings particularly including titanium dioxide nano-coating. As might be expected, the present invention will teach a solution to this production problem.
However, the present invention extends further, it having been recognized that photocatalytic coatingsxe2x80x94howsoever inexpensively obtainedxe2x80x94may be beneficially applied in a manner distinguished over the prior art.
The prior art for the application of photocatalytic coatings of any type basically shows a substantially even, uniform and homogeneous application of these coatings, mostly in the form of solutions that are applied to surfaces in the manner of paint. The present invention will soon be seen to teach otherwise, and to teach that photocatalytic materials are usefully unevenly applied so as to create xe2x80x9chot spotsxe2x80x9d of photocatalytic activity, even if and when the xe2x80x9chot spotsxe2x80x9d are quite small, having dimensions on the order of molecules, and occasionally widely dispersed.
2.2 Prior Art Regarding the Direct Incorporation of Photocatalytic Materials In Other Materials for Anti-fouling Purposes
Photocatalytic titanium oxides have been the focus of several efforts to introduce antifouling properties to coatings and masonry. Examples include Japanese Patent 11 228 204 xe2x80x9cCement composition containing photocatalyst and construction method using itxe2x80x9d; Japanese Patent 11 061 042 xe2x80x9cHighly hydrophilic inorganic coatings, coated products therefrom and their usesxe2x80x9d; and European Patent EP-A885 857 xe2x80x9cUse of a mixture of organic additives for the preparation of cementitious compositions with constant color, and dry premixes and cementitious compositions containing the mixturexe2x80x9d. Wide-spread commercial use has been limited largely due to the relatively high cost and poor dispersion characteristics of commercially available photocatalytic titanium oxide powders. Using photocatalytic titanium oxide is attractive for an anti-fouling product because titanium oxides exhibit robust weatherability and low toxicity. The anatase crystalline form of titanium dioxide exhibits high photocatalytic activity and has been the most widely explored. A problem has been to introduce enough anatase titanium dioxide into the coating or surface formulation to impart anti-fouling properties while maintaining an economic advantage over commercially available leaching-type biocides.
While prior art techniques attempt to minimize cost barriers, they are deficient in one or more areas. For example, extenders have been added to paint formulations to space photocatalyst particles to preserve photocatalytic efficiency, however, these extenders are difficult to distribute within the paint matrix to maximize photocatalytic efficiency. Extenders are typically larger particles and/or in the form of aggregates and thus tend to increase the effective photocatalyst volume concentration and decrease photoactive efficiency as they are added to replace paint resin content, a phenomena analogous to decreasing scattering efficiency as described in F. Stieg, xe2x80x9cThe Effect of Extenders on the Hiding Power of Titanium Pigmentsxe2x80x9d, Official Digest, 1959, pp. 52-64.
Titanium oxide particles, especially anatase titanium dioxide, are difficult to distribute evenly in coating formulations. Anatase titanium dioxide preferentially agglomerates due to a relatively large Hamaker constant (6xc3x9710xe2x88x9220  J) that causes individual photocatalyzing particles to clump and effectively shade each other, reducing photocatalytic efficiency. It would be desirable for photocatalytic particles to disperse more easily in slurries and coating formulations.
A common strategy for improving the dispersion of pigmentary titanium dioxide is to prepare a composite pigment. U.S. Pat. No. 5,755,870 to Ravishankar provides a review of such strategies the teachings of which are incorporated herein by reference. However, the composite pigments described do not attempt to maximize photocatalytic activity and indeed often subdue photocatalysis as a way to protect paint resin from photodegradation.
There is a need for a commercially viable photoactive antifoulant composition that exhibits high photocatalytic activity and disperses easily in slurries and coating formulations.
The present invention contemplates the (i) production and (ii) application, including at industrial scale, of nanoparticulate titanium dioxide (TiO2), and a sol, suitably used as a coating, made of such nanoparticulate TiO2.
The present invention further contemplates composite photocatalytic materials. The preferred materials consist of (1) bodies, most preferably in the form of carrier particles, made of material that is non-photocatalytic and non-interfering with photocatalytically-induced reactions. These (1) bodies have (2) surfaces that are photocatalytic, ergo composite photocatalytic materials.
The present invention still further contemplates highly photocatalytic aggregate particles comprised of an extender particle with discrete photocatalytic titanium oxide particles exposed on the surface. The aggregates may be used as additives for making non-toxic, antifouling coatings and building materials. This invention also includes building materials containing these aggregates and processes for making the aggregates and slurries of the aggregates.
1. Production and Application of Nanoparticulate Titanium Dioxide TiO2) Coating
In its aspect concerning the production of nanoparticulate titanium dioxide (TiO2), and the use of such TiO2 in a sol and as a coating, the preferred particle size distribution of the nanoparticulate titanium dioxide (TiO2) is between 1 nm to 100 nm (as determined from scanning electron microscopy) with less than 0.1 wt. % of carbon in the form of organic compounds or residues. Prior to application, the nanoparticulate TiO2 coating has a particle size distribution of between 1 nm to 100 nm as determined from the absorption onset, a quantum size effect measurement as described in C. Kormann et al., J. Phys. Chem. 92, 5196 (1988), and a transparency of at least 99% measured in a 5 wt. % aqueous/hydrochloric acid solution between 400 and 700 nm in 180xc2x0/d geometry at a layer thickness of 10 xcexcm. xe2x80x9cMonodispersexe2x80x9d means that the collective particles typically have a range of maximum dimension, or diameter, that varies by less than a factor of ten (xc3x9710), and the collective particles will more typically less than a two times (xc3x972) variation in size. Although not at all necessary for their photocatalytic action, and not absolutely necessary for the formation of a sol and the use of same as a coating, it becomes increasingly harder to get uniform quality results with wide variations in the TiO2 starting material, and to that extent some homogeneity is preferred.
The (nanoparticulate) particles of titanium dioxide (within the coating according to the invention) may also be themselves coated with 0.1 to 1000 wt. %, preferably with 5 to 200 wt. %, relative to the TiO2, of at least one oxide, hydroxide or hydrous oxide compound of aluminum, silicon, zirconium, tin, magnesium, zinc, cerium and phosphorus.
The present invention also contemplates a transparent titanium dioxide nanoparticulate liquid coating containing (i) a sol-forming medium and (ii) a sol-forming amount, not exceeding about 20 wt. %, of the nanoparticulate titanium dioxide in accordance with (other aspects of) the invention. The sol-forming medium preferably comprises (i) water, (ii) an alcohol containing 1 to 10 carbon atoms and at least one hydroxide group per molecule, or (iii) a mixture thereof.
1.1 Process for the Production of Nanoparticulate Titanium Dioxide, and a Sol Suitably Used as a Coating
Therefore, in one of its aspects the present invention is embodied in a process for the production of the nanoparticulate titanium dioxide (TiO2), from which TiO2 may be produced a sol suitably used as a coating.
In the preferred process (i) an alkaline-reacting liquid is mixed with (ii) an aqueous solution of titanyl sulfate, optionally containing sulfuric acid, at elevated temperature until the resultant mixture reacts acidically and is neutralized to a pH of approximately between 5 and 9, and more preferably approximately 6.5-7.5, forming (or precipitating) flocculates of titanium dioxide nanoparticles.
The mixture obtained is cooled. The resulting titanium dioxide flocculate formed is isolated through separation by filtration or some other method conventionally recognized in the art, with the isolated nanoparticulate flocculate washed in water and then isolated again. This water-washing step is important. Maximum dispersion into a sol, as will next be discussed, cannot be obtained but that the titanium dioxide nanoparticulate flocculate is first washed in water (before being washed in an acid or alkali, immediately next discussed).
The isolated and water-washed nanoparticulate flocculate is then washed in an acid or an alkali, isolating as a product an acidic or alkaline titania concentrated slurry or cake.
This isolated titania concentrate is dispersed in a polar sol-forming medium to make a sol that is suitable as a coating. The sol is distinguished by, inter alia, being transparent. The sol also beneficially contains less than 0.1 wt. % of carbon, which is as good as or better than any titania sol of the prior art. Finally, this sol will prove to have some very interesting properties, immediately next discussed, when it is applied to a surface.
The transparent titania sol is suitable for application to a surface, including the surfaces of powders or of granules. After being coated with the sol, the surface may optionally be prepared by neutralizing with the required acidic or alkaline reacting compound, and subsequent washing with water. Notably, and importantly, neither the titania concentrate nor the TiO2 of which it is comprised end up on the surface at anything like uniformity at the molecular level. Instead, the titania concentrate, or TiO2, becomes applied to the surface as independent nanoparticles or small agglomerations of nanoparticles, or spots, or islands, that are in size and number dependent upon (i) the density of the titania concentrate in the sol and (ii) the area coated. These nanoparticles, or spots, or islands, are commonly widely separated relative to their own size. Although this uniformity might initially be perceived to be an undesired condition, it is in fact beneficialxe2x80x94see the next section 2.
After being coated with the sol, the surface may further optionally be coated with 0.1 to 1,000 wt. %, and more preferably with 5 to 200 wt. %, relative to TiO2, of at least one oxide, hydroxide or hydrous oxide compound of aluminum, silicon, zirconium, tin, magnesium, zinc, cerium and phosphorus. The surface is still further optionally (i) dried and/or (ii) annealed.
The polar sol-forming medium preferably comprises water, an alcohol containing 1 to 10 carbon atoms and at least one hydroxide group per molecule, or a mixture thereof.
Perhaps surprisingly, the nanoparticulate TiO2 coating according to the invention may be successfully produced within a large scale industrial process, namely TiO2 pigment production using the sulfate process, and is thus very simple and economically viable.
The filter residue obtained (after the washings) and the coating obtained (after application of the sol film) using the process according to the invention may be inorganically and/or organically post-treated.
In principle, any aqueous titanyl sulfate solution is suitable as the educt. Said solution may optionally contain sulfuric acid. Contamination by metals which form soluble sulfates and chlorides, such as for example iron, magnesium, aluminum and alkali metals do not in principle disrupt the production process, unless the stated elements have a disadvantageous effect even in trace quantities in the intended application. It is thus possible to perform the process according the invention on a large industrial scale. Black liquor, as is obtained from the sulfate process by digesting ilmenite and/or titanium slag with sulfuric acid, dissolving the resultant digestion cake in water and performing clarification, may for example be used as the educt.
The production process according to the invention is, however, not restricted to black liquor as the educt. Examples of other processes for the production of titanyl sulfate solution suitable as an educt include:
1) dissolution of commercial grade titanyl sulfate in water;
2) dissolution/digestion of titanium dioxide and TiO2 hydrates, for example orthotitanic acid, metatitanic acid, in H2SO4;
3) dissolution/digestion of alkali metal and magnesium titanates, also in hydrous form, in H2SO4;
4) reaction of TiCl4 with H2SO4 to form TiOSO4 and HCl, as described in DE-A 4 216 122.
The products, in particular those from 1), 2) and 3), are preferably used as titanyl sulfate solutions when traces of foreign metals (for example iron) are not desired in the product according to the invention.
In order to achieve economically viable operation, the titanyl sulfate solutions to be used according to the invention preferably contain 100 to 300, and more particularly preferably 170 to 230 g of titanium/l, calculated as TiO2.
Aqueous solutions of ammonium hydroxide, sodium hydroxide, or potassium hydroxide are preferably used as the alkaline-reacting liquid; it is, in principle, also possible to use carbonates of sodium, potassium and ammonium, but these are less suitable due to vigorous evolution of CO2. Ammonium hydroxide solution is particularly preferred as sodium and potassium ions are not introduced as a contaminant and is used to illustrate performance of the process in greater detail.
The quantity of ammonia should be calculated such that the reaction medium at the end of step a) has a final pH of approximately between 5 and 9, and more preferably between 6.5 and 7.5.
The ammonia is preferably used as an ammonium hydroxide solution having a concentration of approximately between 1 to 8 molar NH4OH and more preferably between 1 to 4 molar NH4OH.
The reaction of ammonium hydroxide solution with the titanyl sulfate solution preferably proceeds in such a manner that the ammonium hydroxide is added to a solution of titanyl sulfate, heated to approximately 60 to 100xc2x0 C.
Preferably the reaction of the ammonium hydroxide and titanyl sulfate solution can also be carried out by adding the two reactants simultaneously and mixing them with stirring at temperatures of between 60 and 100xc2x0 C.
This reaction of the titanyl sulfate solution should preferably be performed with vigorous stirring and at temperatures of 60 to 100xc2x0 C.
The addition of the ammonium hydroxide to the titanyl sulfate solution should preferably take no longer than 30 minutes.
Once reacted, the resultant mixture should preferably be quenched to temperatures of below 60xc2x0 C. and then optionally stirred for xc2xc to 1 hour at this temperature.
In summary, the production of the sol suitable as a coating, and the sol so produced, has myriad, and distinguishing, advantages. The sol is uniquely transparent while achieving the desirably low carbon of the best prior art titania sols. The yield in making the sol is unexcelled; virtually 100% of the precipitated titanium flocculates are taken up into the sol. The process of making the sol is readily scalable to industrial scale. Finally, and as a seemingly subtle differentiation in the sol the use and benefit of which is unanticipated in the prior art, the sol, when used as a coating, will not deposit its titanium dioxide uniformly (upon a coated surface, which may be a particle) but will instead lay down the titanium dioxide in microparticles, or spots, or islands. The very significant advantage of this is immediately next discussed in section 2.
2. Composite Photocatalytic Materials
In its aspect concerning the realization of composite photocatalytic materials, the preferred material of the present invention includes, as previously stated, (1) bodies that are most preferably in the form of carrier particles and that are made of material that do not interfere with photocatalytic activity and do not adversely interact with other components in an end-use application. These (1) bodies that are non-deleterious to photocatalytic reaction have (2) surfaces that are photocatalytic, forming thus a composite photocatalytic material.
Moreover, these (2) surfaces are not substantially evenly possessed of photocatalytic material and photocatalytic action, but preferably have such photocatalytic material highly specifically located in xe2x80x9cspotsxe2x80x9d, or xe2x80x9cislandsxe2x80x9d that may themselves be either 2-or 3-dimensional.
To realize these xe2x80x9cislandsxe2x80x9d of photocatalyst, the (2) surfaces of the (1) bodies, or carrier particles, are not made from continuous films of photocatalytic material, but are instead made by attaching discrete nanoparticles of photocatalyst. These nanoparticles of photocatalyst are preferably smallerxe2x80x94normally 1xc3x9710-9 to 1xc3x9710-7 in diameterxe2x80x94than are the carrier particles themselves, which are commonly about 1xc3x9710-7 to 1xc3x9710-2 meters in diameter, depending on application.
Both the size of the (2) carrier particles, or bodies, and the density of the spots, or islands, of (1) surface photocatalytic material are a function of intended application. An exemplary application of a carrier large particle might be for use in a gravel-like roof coating where it is substantially desired only that large, ground-observable, patches of algae should not grow on the roof. In this application the photocatalytic spots, or islands, might also be relatively widely separated, the main goal not being to kill every bacteria or algal cell on the roof, but to prevent formation of a bio-film. Exemplary applications of small carrier particles include the lips of a swimming pools, bathroom tiles, and hospital coatings where it is desired to avoid all bacterial growth whatsoever. Not only are the carrier particles small, but the photocatalytic spots, or islands, may be relatively close spaced (although normally not continuous).
As an aside, the photocatalyst of the present invention is generally not intended for use in liquids other than coatings, and certainly not for antiseptic solutions where photocatalyst suspensions kill microbes or algae on surfaces. The only time the inventor has used photocatalyst suspensions was in lab tests wherein algae was suspended in water and photocatalyst particles were then introduced into the water to see xe2x80x9cfor a first glimpsexe2x80x9d whether the photocatalyst killed the algae. However, it is contemplated that the photocatalyst of the present invention could be dispersed in water to destroy microbial suspensions. One such application could be to destroy harmful algae blooms in lakes and bays. The three main benefits of using photocatalyst of the present invention in natural waterways would be (i) low toxicity to higher life forms, (ii) limited persistence in the environment (the concentrated contaminants of natural water systems tend to foul the photocatalyst, inactivating it over time), and (iii) excellent dispersion properties in water (in contrast to poor dispersion for virgin photocatalyst).
Accordingly, by incorporating but minute amounts of dispersed photocatalytic nanoparticles solely upon the surfaces of carrier particlesxe2x80x94most typically in an amount of less than 20% and more typically 5% by weight in the composite materialxe2x80x94these dispersed photocatalytic nanoparticles, and diverse surfaces coated with the composite material, are highly effective in killing microorganisms, including both algae and bacteria, in the presence of light in the visible or ultraviolet wavelengths. Indeed, by attaching microparticles of preferred photocatalytic materials of titanium dioxide, zinc oxide and tungsten oxide and mixtures thereof onto the surface of particles of silicate and carbonate powders and sands, mineral and mineral composites, inorganic pigments, construction aggregates, polymers and like common materials in an amount of less than 10% by weight, the composite particle""s so formed are at least 50% as effective in killing algae and bacteria as are the pure photocatalysts themselves. Accordingly, there is at least a five-to-one (5:1), and more typically a twenty-to-one (20:1), gain in efficiency in the usage of the photocatalytic materialsxe2x80x94which are greatly more expensive than are the materials from which the carrier particles are made.
The composite photocatalytic materials, preferably particulate materials, may themselves be combined with any of dispersants, carriers, binders and the like to make any of aqueous solutions, coatings, paints and the like as exhibit any of algicidal, fungicidal, and/or anti-bacterial effects. Liquids, aggregates and solids incorporating the composite photocatalytic materials of the present invention may be, for example, coated or painted onto, by way of example, the interior and exterior surfaces of buildings and swimming pools.
Although no theory of the operation of the composite photocatalytic materials of the present invention is necessary to make these materials, nor to take advantage of their operational characteristics, it is possible to speculate on the operation of the materials of the present invention. It is hypothesized that only a minute microparticle of pure photocatalytic material such as titanium dioxide, zinc oxide and tungsten oxide and mixtures thereof is necessary to adversely affect a much larger bacterium, or a cell of an algae; that it is not the total amount of photocatalyst that does the damage to lower life forms, but the manner in which a photocatalyst is deployed against these life forms.
Apparently it is not necessary for control of simple life forms to expose in the presence of light the entirety of the life form to a photocatalyst in order to enjoy a prophylactic effect. It is apparently sufficient for a prophylactic effect to expose only a minute region of the life form. It may even be the case that a bacterium or an algae will retreat from an extensive area of photocatalyst with less damage than it will sustain when exposed, hypothetically for a longer time, to but a microscopic spot, or particle, or photocatalyst to which its primitive sensory system is insufficiently sensitive. The present invention suggests that large surfaces, such as walls of swimming pools and buildings, should not have photocatalyst evenly applied so that, at some density of adjacent bacterial or algal life forms, a bio-film will be formed, the photocatalyst overwhelmed (including by occlusion of light energy), and the surface populated. Instead, it may be preferable that the surface act as a xe2x80x9ctrojan horsexe2x80x9d, according areas devoid of photocatalystxe2x80x94which areas are sufficient in size to be populated by one or a few bacteria or algal cells until these bacteria or algae grow and/or reproduce, forcing members of the incipient community into damaging contact with minute regions of photocatalyst. These minute regions, or microdots, or microparticles, of photocatalyst may, at their high concentrations, be very effective in promoting electron exchange in the presence of impinging light. They may become xe2x80x9chot spotsxe2x80x9d of xe2x80x9cstingingxe2x80x9d death to those microorganisms with which they come into contact.
The mechanism(s) of photocatalytically-induced fungicidal, bacteriocidal and like effects are poorly understood, but the present invention suggests that there is more to the conservative and focused deployment of photocatalysts than simply saving money by minimizing usage. The present invention suggests that photocatalyst should be parsimoniously used as a microbial rapierxe2x80x94the point of which can be deadly to microbial lifexe2x80x94instead of as a bludgeon by which the substantial surface of a microbe is substantially evenly irritated in a manner that may not prove fatal to the microbe.
2.1 A Composite Photocatalytic Material
Accordingly, in another of its aspects the present invention is embodied in a composite body exhibiting a photocatalytic effect. The body has (i) a core consisting essentially of a material without deleterious photocatalytic effect on the composite body nor adverse interaction with other components in an end-use application, and (ii) a photocatalytic material upon the surface of the core. This photocatalytic material is less than 20% by weight of the combined photocatalytic material and the core.
The core is a preferably a particle, and more preferably a particle of less than 1 (one) centimeter in diameter. Meanwhile, the photocatalytic material is preferably a multiplicity of particles each of which is preferably of diameter less than one hundred (100) nanometers. By this construction the composite body is also a particle.
The core preferably consists essentially of a material, nondeleterious to photocatalytic reactions, drawn from the group consisting of silicates and carbonates, mineral and mineral composites, metal oxides, inorganic pigments, and construction aggregates. Alternatively, the core may consist essentially of a polymer. The polymer core is preferably drawn from the group consisting essentially of acrylics, acrylonitriles, acrylamides, butenes, epoxies, fluoropolymers, melamines, methacrylates, nylons, phenolics, polyamids, polyamines, polyesters, polyethylenes, polypropylenes, polysulfides, polyurethanes, silicones, styrenes, terephthalates, vinyls.
The photocatalytic material is preferably drawn from the group of metal compound semiconductors consisting essentially of titanium, zinc, tungsten and iron, and oxides of titanium, zinc, tungsten and iron, and strontium titanates. This compound semiconductor photocatalytic material may be combined with a metal or metal compound drawn from the group consisting of nickel, cobalt, zinc, palladium, platinum, silver, and gold. Most preferably, the photocatalytic material is drawn from the group of metal compound semiconductors consisting essentially of anatase titanium dioxide and zinc oxide.
The composite photocatalytic material is preferably in the form of particles having a diameter from 100 nanometers to 1 centimeter, which diameter depends upon the core size selected and the intended end-use application.
The weight of the photocatalytic material is preferably less than 20% of the weight of the core, and more preferably less than 10% of the weight of the core.
The composite photocatalytic material in accordance with the present invention is usefully incorporated in other compositions. When so incorporated, it is preferably so incorporated in amounts from 0.001% to 85% by volume. The composite photocatalytic material may be incorporated with, or on, one or more materials from the group of building materials consisting of concrete, cement, stucco, masonry, roofing shingles, wall shingles, building siding, flooring materials and swimming pool surfaces. The composite photocatalytic material may be incorporated in a composition that is effective as an anti-fouling coating. For example, it may be incorporated in a concrete coating effective in killing by contact algae, fungus and/or bacteria on surfaces.
Most typically, at a proportion by weight in the composite particle of less than 10%, the efficacy of the photocatalytic material within the composite particles to kill by contact both algae and bacteria upon surfaces is at least one-half (0.5) as good as is the efficacy of this same photocatalytic material in purest form to kill. In other words, at least equal killing effect is realized with at least a five to one (5:1) reduction in the amount of photocatalytic material used (when this photocatalytic material is upon the surface of the composite particles).
2.2 Methods of Making Composite Photocatalytic Particles
In yet another of its aspects (concerning the making and use of photocatalytic materials), the present invention is embodied in methods of making composite photocatalytic particles.
In one method an aqueous slurry of first particlesxe2x80x94these particles consisting essentially of a material without deleterious photocatalytic effect on the composite particle nor adverse interaction with other components in an end-use application, and having a size in the range from 100 nanometers to 1 centimeter diameterxe2x80x94is prepared.
To this slurry is added a colloidal suspension of 0.1% to 60% by weight second particles, which second particles consist essentially of photocatalytic material having diameters in the range from 1 to 100 nanometers. The combined weight of second particles in the colloidal suspension is less than 20%, and more preferably less than 10%, of the combined weight of the first particles that are within the aqueous slurry.
The aqueous slurry and the colloidal suspension is mixed so that the photocatalytic material second particles attach through van der Waals forces or chemical fusion to the nondeleterious material first particles, forming a slurry of composite particles. In these composite particles the relatively smaller photocatalytic material second particles are located upon the surfaces of the relatively larger, nondeleterious material, first particles.
The photocatalytic material is in weight preferably less than 20%, and more preferably less than 10%, of the first particles. The added colloidal suspension added is preferably from 0.1% to 60% by weight second particles. The colloidal suspension added is preferably of the highest solids concentration at which the suspension is stable, normally being in the range from 14% to 50% by weight.
The pH of the mixing is often beneficially adjusted so that both the photocatalytic material second particles and the nondeleterious material first particles are displaced to the same directionxe2x80x94whether above or belowxe2x80x94from their respective isoelectric points (those points at which the particles have a neutral net charge). Furthermore, the nondeleterious material first particles and the photocatalytic material second particles may also have opposite charge.
The adding of the colloidal suspension of second particles, or the mixing of the aqueous slurry and the colloidal suspension, or both the adding and the mixing, may optionally transpire in the presence of at least one dispersant.
The method may continue with one or more well-known finishing steps such as filter, wash and/or dry the composite photocatalytic particles.
When the aggregation of composite photocatalytic particles is dried, composite particles with heat resistant cores are then preferably annealed in a kiln to create stronger fusion bonds between the photocatalytic material second particles and the nondeleterious material first particles and/or to improve the photocatalytic nature of the photocatalyst by changing its crystalline form. Moreover, the annealed composite photocatalytic particles are preferably rapidly cooled to ambient room temperature; this may be simply accomplished by removing the hot material from the kiln to facilitate heat transfer away from the material. The time period of this cooling is necessarily dependent, at least in part, upon the temperature of the annealing and the amount of the composite photocatalytic particles. However, it is preferably less than six hours. Since this forced rapid cooling might normally be considered to induce fracturing in metals, it is uncommonly applied to the materials (including metal oxides) of the present invention. However, it has benefit in that it increases photocatalytic activity.
3. Photocatalytic Aggregate Particles
In still yet another of its aspects, the present invention contemplates highly photocatalytic aggregate particles comprised of an extender particle with discrete photocatalytic titanium oxide particles exposed on the surface. The extender particle reduces the amount of premium photocatalyst required to achieve desired photocatalytic activity in a finished product. The discrete nature of the photocatalytic titanium oxide particles, applied in sufficient number, increases the photoactivity of the aggregate particles by increasing their photoactive surface area verses the surface area provided by a relatively flat continuous coating. The aggregates of this invention exhibit an inhibitory effect on surface-borne microorganisms when the mixtures are incorporated into building materials such as masonry, roofing shingles, siding, and antifouling coatings. Further, the aggregate particles show improved handling and dispersion in coating preparations versus virgin photocatalyst.
The invention also contemplates processes for making such aggregates, slurries of the aggregates, coatings, building materials, and masonry containing the aggregates.
3.1 The Preferred Photocatalytic Aggregates
The preferred aggregate particles of the present inventionxe2x80x94generally comprised of an extender particle with discrete photocatalytic titanium oxide particles exposed on the surface, which exhibit antifouling properties and improved dispersion in slurries and coatingsxe2x80x94consist essentially of photocatalytic titanium oxide, preferably titanium dioxide in the anatase crystalline form, at less than about 20% by weight, preferably less than 10% by weight, and more preferably less than 6% by weight, and an extender particle at greater than 20% by weight. Preferred extender particles include silicate and carbonate powders, mineral and mineral composites including calcined clay and wollastonite, metal oxides including zinc oxide, inorganic pigments, and construction aggregates including roofing granules.
In one preferred embodiment, colloidal anatase titanium dioxide in an amount less than 6 weight % is dispersed on the surface of crystalline silica powder having an average particle diameter of 0.7 to 5 microns. In another preferred embodiment, colloidal anatase titanium dioxide in an amount less than 6 weight % is dispersed on the surface of zinc oxide powder having an average particle diameter of 0.7 to 5 microns.
This invention also includes anti-fouling building products, including coatings and masonry compositions, comprising aggregate photocatalytic particles of this invention at a volume concentration of 0.001% to 85% where the anti-fouling coatings and masonry resist the growth of microorganisms when U.V. or visible light energy is present to activate the aggregate photocatalytic particles. Building products include roofing granules, roofing shingles, building siding, wall shingles, hard flooring, and swimming pool surfaces.
3.2 Preferred Processes for Producing Photocatalytic Aggregates
Several different processes for making the above-described aggregate photocatalytic materials are preferred. In one embodiment, an aqueous slurry of extender particles are mixed with a solution of titanyl sulfate and by the addition of an alkaline reacting agent, discrete titanium dioxide particles are deposited onto the extender particles.
In another embodiment, an alkaline or acidic titania sol is mixed with extender particles where the particles in the titania sol have an average diameter size within the range of about 1 to about 100 nanometers. The solution is maintained such that the extender particles and the sol particles are both above or below their respective isoelectric points such that substantially discrete particles of titanium dioxide are dispersed onto the surfaces of the extender particles in an amount less than 20 weight % based on aggregate particle weight.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.